Question: The arithmetic sequence $(a_i)$ is defined by the formula: $a_i = -9 + 4(i - 1)$ What is $a_{20}$, the twentieth term in the sequence?
Explanation: From the given formula, we can see that the first term of the sequence is $-9$ and the common difference is $4$ To find $a_{20}$ , we can simply substitute $i = 20$ into the given formula. Therefore, the twentieth term is equal to $a_{20} = -9 + 4 (20 - 1) = 67$.